TN295 



No. 9079 






40 • 








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IC 9079 



Bureau of Mines Information Circular/1986 



Improved Backup Alarm Technology 
for Mobile Mining Equipment 



By Guy A. Johnson, Russel 
and Linneas W. Laage 



E. Griffin, 



(^ 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 9079 

It 



Improved Backup Alarm Technology 
for Mobile Mining Equipment 



By Guy A. Johnson, Russell E. Griffin, 
and Linneas W. Laage 




UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 




ins 

.114 



Library of Congress Cataloging in Publication Data: 



Johnson, Guy A 

Improved backup alarm technology for mobile mining equipment. 

(Information circular ; 9079) 

Supt. of Docs, no.: I 28.27: 9079. 

1. Mining machinery —Safety measures. I. Griffin, Russell E. 
II. Laage, Linneas W. III. Title. IV. Series: Information circular 
(United States. Bureau of Mines) ; 9079. 



TN295.U4 [TN3451 622s [622\8] 86-600017 



CONTENTS 

Page 

Abstract 1 

Notice 2 

Introduction 3 

General principles — design criteria 4 

Proof-of-concept testing of prototype systems 5 

Infrared systems tested 6 

STI Omniprox 3070 6 

Search-Eye 10 

Ultrasonic sensing system 10 

Doppler radar system 13 

Possible hardware improvements 15 

installation tips 17 

Discussion 17 

Infrared systems 17 

Ultrasonic sensing systems 18 

Doppler radar systems 18 

Conclusion 18 

Appendix. — Sources of equipment « 19 

ILLUSTRATIONS 

1 . Discriminating backup alarm design 4 

2 . Ideal detection zone 5 

3. Infrared sensor mounted on front-end loader 7 

4. Infrared electronic control card 7 

5. Infrared sensor on a Terex 90C 8 

6. Close-up of infrared system sensor on a Terex 90C 8 

7. Infrared sensor system on a Clark 275 9 

8. Infrared system on a Caterpillar 980 9 

9. Schematic diagram of an ultrasonic detection system 11 

10. Ultrasonic detection system installed on a Clark 275C 12 

11. Close-up of ultrasonic sensors 12 

12. Measuring the detection zone 13 

13. Close-up of a Doppler radar detection system 14 

14. Detection zone of a Doppler radar system 15 

15. Testing a Doppler radar system with an anthropomorphic dummy 16 

16. Prototype synthesized voice warning system.... 16 

TABLES 

1. In-mine tests of discriminating backup alarm systems 6 

2. Characteristics of backup alarm systems tested 6 





UNIT OF MEASURE ABBREVIATIONS USED 


IN THIS REPORT 


ft 


foot rpm 


revolution per minute 


ft2 


square foot st 


short ton 


g 


gram V 


volt 


GHz 


gigacycle per second V ac 


volt, alternating current 


h 


hour V dc 


volt, direct current 


m 


meter yd 


yard 


ms 


millisecond yd 3 


cubic yard 


mW 


milliwatt yr 


year 


pet 


percent 





IMPROVED BACKUP ALARM TECHNOLOGY FOR MOBILE MINING EQUIPMENT 

By Guy A. Johnson, Russell E. Griff in, and Linneas W. Laage 



ABSTRACT 

Despite the use of warning alarms to alert miners to the backward 
movement of large mining equipment, miners still are injured too fre- 
quently in backup accidents. Currently approved backup alarm technology 
consists of continuous sounding alarm systems. New technology has de- 
veloped a warning system that initiates an alarm only if there is an 
object close behind the vehicle. This advancement eliminates constant 
exposure to the alarm because a warning is given only in case of a po- 
tential collision. Adoption of this development can improve safety and 
reduce damage, especially for front-end loaders (FEL's). It will also 
eliminate a source of nuisance noise in urban and residential areas. 

This Bureau of Mines report describes the general characteristics of 
infrared (IR) light, ultrasonic wave, and Doppler radar technology as 
used for backup alarms, and reviews the testing of the most promising 
detection devices. 



Supervisory mining engineer (now with Denver Research Center, Bureau of Mines, 
Denver, CO. ) . 

^Electronics engineer. 
^Mining engineer. 
Twin Cities Research Center, Bureau of Mines, Minneapolis, MN. 



NOTICE 
IMPROVED BACKUP ALARM TECHNOLOGY CAN BE APPLIED IN TWO WAYS 

1. As a supplement to conventional backup alarms by sounding an additional 
alarm in the cab when a collision hazard is detected. 

2. As a switch for the conventional backup alarm, sounding the alarm only 
when a collision hazard is present. With conventional backup alarms the oper- 
ator is expected to work the machine despite the alarm. With the improved 
backup alarm both the potential victim and the operator can respond to avert a 
collision because the false alarm aspect is eliminated. 



CAUTION 

The following statement was prepared by the Director, Office of Standards, 
Regulations, and Variances, Mine Safety and Health Administration, for instal- 
lations where the conventional backup alarm is switched on by the improved 
backup alarm (method 2 above). 

"Since the discriminating backup alarm does not give an automatic warning 
when the mine machine is put in reverse, it cannot be used to satisfy existing 
Federal requirements unless a petition for modification has been issued by the 
Mine Safety and Health Administration; therefore, anyone who would like to use 
the device will have to file a petition. This petition must be in writing to 
the Assistant Secretary of Labor for Mine Safety and Health." 

"The petition must contain the name and address of the petitioner; mailing 
address and identification number of the mine or mines affected; the mandatory 
safety standard to which the petition is directed and a concise statement of 
facts that would warrant the proposed modification." 



CAUTION 

These systems must be installed and aimed properly to detect collision 
hazards. 



These systems must be regularly inspected for operation and performance. 



INTRODUCTION 



The Mine Safety and Health Administra- 
tion (MSHA) (30 CFR 56.9087, 57.9087, and 
77.410) requires "an automatic reverse 
signal alarm" on mobile surface mining 
equipment. These alarms usually are loud 
horns or bells on the rear of the equip- 
ment which are activated and in continu- 
ous operation when the vehicle's trans- 
mission is shifted into reverse. Their 
purpose is to warn miners of the rearward 
movement of the vehicle. 

As part of its research to develop col- 
lision protection technology for larger 
mobile mining equipment, the Bureau of 
Mines analyzed the occurrence of backup 
accidents. Bureau investigators reviewed 
fatality reports and held informal dis- 
cussions with such safety organizations 
as the Lake Superior Mines Safety Coun- 
cil, the Association of Arizona Mine 
Safety Engineers, the Wyoming Chapter of 
the American Society of Safety Engineers, 
and the Safety Section of the National 
Sand and Gravel Association. Results of 
this study show that back-over accidents 
still occur regularly, apparently because 
miners can become "immune" to the sound 
of current backup alarms . 

Semi continuous backup alarms inevitably 
produce the effect of a "false alarm," 
and repeated false alarms desensitize 
people exposed to them. The U.S. Army 
recognized this in Military Standard 
1472: "The design of audio display de- 
vices and circuits shall preclude false 
alarms . "4 

In mines, workers are constantly ex- 
posed to the repeated sounds of many 
backup alarms on various pieces of equip- 
ment. Consequently, they tend to pay 
little attention to any one specific 
warning. This is especially true for 
maintenance and ground crew personnel who 
constantly work near FEL's. 

To counteract this problem, the Bureau 
of Mines has adapted and developed sen- 
sors that will automatically turn on a 
backup alarm only when some object 

^U.S. Army. Human Engineering Design 
Criteria for Military Systems, Equipment 
and Facilities. Military Standard 1472C, 
1984, p. 51. 



(collision hazard) is behind a vehicle at 
a distance of 15 to 20 ft. If a warning 
is given only when the danger is real, it 
will get a greater response both from 
miners on the ground and from equipment 
operators. 

The Bureau reviewed the literature to 
determine the maximum proximity range at 
which sensors could be made rugged enough 
to be reliable in mines at a reasonable 
cost. Several options were found. Re- 
cent advancements in microcircuitry tech- 
nology make it easy to fabricate the 
electronic components for area and driver 
warnings once the proper detection sen- 
sors have been selected. 

Military-type detectors were studied 
first and found to be reliable but very 
expensive. Viability and low cost were 
both essential, and the fast-growing 
field of security and intrusion detectors 
offered promise. The security field has 
created a large market for sensors that 
give close-in object detection; for exam- 
ple, a sensor with the ability to detect 
the presence of a person sneaking across 
a room. Because of the economics of 
scale inherent in this market, relatively 
sophisticated sensing technology is now 
available at a very reasonable price. 

In the early 1980' s, the Bureau began 
testing, first in the laboratory and then 
in the field, prototype devices with a 
potential for solving the backup colli- 
sion problem. Although improvements in 
mirrors, the development of blind area 
viewers, and advancements in closed cir- 
cuit television already had made it 
easier for the operators of large equip- 
ment to see potential hazards, alarms 
still were needed to direct the attention 
of drivers, and potential victims, to 
specific dangers. 

In addition to solving the "false 
alarm" problem, an alarm that sounds only 
when a hazard exists can help to minimize 
the nuisance noise that extends to a 
mine's surrounding environment. Such 
noise is especially irritating in the 
early morning and evening hours around 
urban crushed stone or sand and gravel 
pits. Even though the miners often do 
not react to the backup alarms , 



people living close to the operations are 
constantly bothered by the noise. Alarms 
that sound only when something or someone 



is behind and close to a vehicle will be 
a boon to both mines and their neighbors. 



GENERAL PRINCIPLES— DESIGN CRITERIA 



In addition to the considerations pre- 
sented above, a few key principles have 
emerged from the Bureau's research into 
improved protection for backing vehicles. 
The "target area" behind the vehicle is 
relatively close, around 15 to 20 ft. 
(See figure 1.) Objects beyond this area 
are not a major hazard because they can 
be seen either in the vehicle's mirror or 
directly by the driver when the backing 
vehicle is turning. The rear-looking 
detector(s) are not just for sensing 
individuals or other vehicles. These 
systems can detect objects as small as 1 
ft 3 (e.g., boulders, stacks of mainte- 
nance materials, etc.) and warn of their 
presence. 

This warning is accomplished when the 
hazard detection system initiates an in- 
cab backup horn or buzzer to alert the 
driver. (See figure 2.) An in-cab buzz- 
er can be added to the system when vehi- 
cle cabs are acoustically isolated from 



the external backup alarm. This in-cab 
warning feature uses modern, miniaturized 
electronic components that are low in 
cost. It is also cost effective because 
it alerts the driver to the possible 
presence of small objects that could dam- 
age the vehicle's tires, thus avoiding 
both the high material cost and lost time 
involved in replacement. 

During the Bureau's research, several 
alternative technologies were examined. 
In 1982 and 1983, after a few simple in- 
frared (IR) obstacle detection devices 
had been tested, a rugged unit produced 
by Scientific Technology, Inc. (STI), 5 
Mountain View, CA, was found to give ade- 
quate coverage and distance during 
laboratory testing. (The IR sensor op- 
tion is discussed in the next section. ) 



-'Reference to specific 
not imply endorsement by 
Mines. 



products does 
the Bureau of 





-Transducer 



Backup alarm 
coverage area 



FIGURE 1.- Discriminating backup alarm design. 




I"-" 



FIGURE 2. - Ideal detection zone. 



Polaroid has packaged its ultrasonic 
ranging transducer (used on its cameras) 
for general use. This device was tested 
on a 24-yd 5 -capacity FEL and a 120- 
st-capacity haulage truck at the Bureau's 
Twin Cities (MN) facility, but the beam 
pattern was found to be too limited in 
diameter for on-vehicle use. (Followup 
work with a more sophisticated, long- 
range, ultrasonic system prototype for 
use on small construction equipment is 
also reported below.) 



The most recent work involves a newly 
developed short-range Doppler radar unit. 
In the late 1970' s, radar collision pro- 
tection was tested but found only practi- 
cal for long-range (100- to 300-ft) ap- 
plications because of signal processing 
circuitry limitations. The new Doppler 
radar systems are designed for close (4- 
ft) collision protection, so increasing 
the range is now a problem. Once the 
hardware modification problems can be 
worked out of this unit, it will be the 
most promising alternative yet tested. 



PROOF-OF-CONCEPT TESTING OF PROTOTYPE SYSTEMS 



The work detailed here is an extension 
of the Bureau's earlier efforts to use 
state-of-the-art technology in solving 
visibility problems inherent in large 



surface equipment. Table 1 summarizes 
the testing of discriminating backup 
alarms. Table 2 summarizes the charac- 
teristics of the backup alarm systems. 



TABLE 1. - In-mine tests of discriminating backup alarm systems 



System type and 


Testing 


Equipment 


System 


Comments 


location 


dates 


tested on 


range, 1 ft 




Infrared: 










Limestone quarry 


Mar. -Sept. 


Terex 90C. 


Up to 40. . . 


System had to be removed 


(MD). 


1983. 






and made more rugged. 


Sand mine (MN) . . 


Oct. -Dec. 
1983. 


Clark 275.. 


Up to 40. . . 


System removed owing to 
light sand-sun reflection 
problems. 


Sand and gravel 


July 1984- 


Caterpillar 


Up to 40. . . 


Marginal usage because of 


pit (CO). 


present. 


980. 




moisture condensing inside 
lens. 


Ultrasonic: 










Limestone quarry 


Mar. 1984- 


Caterpillar 


Up to 17... 


Working well. 


(MD). 


June 1985. 


988B. 






Sand mine (MN).. 


May 1984- 
present. 


Clark 275C. 


Up to 17.. . 


Working well. 


Doppler radar: 










Sand and gravel 


Mar. 1985- 


Caterpillar 


Up to 18... 


Initial interference be- 


pit (MO). 


present. 


992. 




tween back-up alarm and 
cab annunciator. Correct- 
ed by a modified unit. 


Sand and gravel 


Feb. 1985 


Caterpillar 


Up to 18. . . 


Switch problem at 


pit (CO). 


(1 day). 


988. 




installation. 


Sand mine (MN) . . 


May 1985- 
present. 


Clark 275C. 


Up to 20. .. 


Maintenance problem, which 
was resolved. 



1 Depending on size and reflectivity. 

TABLE 2. - Characteristics of backup alarm systems tested 



System type and manufacturer 



Shape of area coverage 
pattern 



Performance affected by — 



Infrared: Scientific Tech- 
nology, Inc. 

Ultrasonic: Global Fabrica- 
tions Co. , Ltd. 

Doppler radar: Con-Serv, Inc, 



2 parallel narrow cones 
extending from sensors, 

11- by 17-ft rectangle. 

12- by 20-ft elongated 
teardrop. 



Sunlight, dust, reflectivity 
of object. 

Airflow, acoustical reflec- 
tivity of object. 

Radar profile of object. 



INFRARED SYSTEMS TESTED 

STI Omniprox 3070 

The STI Omniprox 3070 series sensor was 
the first of the IR type to be laboratory 
and field tested. This sensor is a 
solid-state, modulated, IR beam detection 
and control device, provided in a modular 
configuration. The sensor head is total- 
ly sealed and shock tested (100 g at 10 
ms). The manufacturer states it can be 



mounted anywhere — indoors or outdoors , 
submerged or in a vacuum — and can be lo- 
cated up to 30.5 m (100 ft) from the con- 
trol electronics. (Figure 3 shows the 
unit mounted on a FEL.) It is capable of 
disregarding ambient light (though direct 
sunlight does affect it) , atmospheric 
contamination, and thin film accumula- 
tions of oil, dust, water, and other air- 
borne deposits. Field tests support 
these latter claims. 





FIGURE 3. - Infrared sensor mounted on front-end 
loader. 



FIGURE 4. - Infrared electronic control card. 



The system is limited in its area of 
coverage. The 3070 series has an adjust- 
able range of up to 3.7 m (12 ft) in the 
proximity mode (target size 12 in) and 18 
m (60 ft) in the retroref lective mode. 
The range sensitivity is adjustable 
through a potentiometer on the control 
electronics. The maximum IR beam is typ- 
ically 2-ft diam at a distance of 20 ft, 
which is too narrow for general in-mine 
use. 

A variety of standard output and con- 
trol options are available to adapt the 
system for different applications. For 
the Bureau tests, however, it was used 
with two sensors with overlapping beams , 
in a logic "or" mode with a double-pole, 
switched-relay output. With this setup, 
either sensor detecting an object can ac- 
tivate a backup alarm, an audio in-cab 
alarm, or a light, in any combination de- 
sired. It is also designed to easily 
adapt to operation on 115 or 230 V ac, 
10.5 to 13 V dc, or 24 V ac or dc. For 



testing purposes, the electronics control 
circuit card was ordered unmounted. It 
was then adapted for 24-V dc operation, 
and mounted in a National Electrical Man- 
ufacturers Association (NEMA) 12, type D 
enclosure, with appropriate holes added 
for power and sensor leads (fig. 4). 

The system was field tested in three 
different locations over a period of sev- 
eral months. The first test was on a 
Terex 90C FEL. At the time, the 90C was 
a preproduction, pilot model machine, us- 
ing an 8- to 11-yd 3 bucket on a field 
trial and demonstration. The sensors 
were mounted on adjustable brackets weld- 
ed to the frame on either side of the 
radiator grille, about 6 ft above ground 
level. (See figures 5 and 6.) The con- 
trol box and annunciator were mounted in 
the cab on a dash panel located to the 
right of the operator. Connecting cables 
between the two, and a cable connecting 
the backup alarm to the control box, were 
run under the loader deck alongside 







FIGURE 5. - Infrared sensor on a Terex 90C. 




FIGURE 6. - Close-up of infrared system sensor on a Terex 90C. 




FIGURE 7. - Infrared sensor system on a Clark 275. 




FIGURE 8. - Infrared system on a Caterpillar 980. 



10 



existing cable and hose runs. The con- 
necting cables for the sensors, which 
consisted of two-pair shielded conductors 
(Belden 8723) , were supplied by the manu- 
facturer. For convenience of installa- 
tion, connectors were inserted in the 
sensor cables approximately 5 ft from the 
sensor. Power was obtained from a ter- 
minal on the oil pressure switch, which 
meant the system would be on if the ve- 
hicle's engine were running. The FEL's 
backup alarm was connected to the system 
to sound if the sensors detected an ob- 
ject in their coverage area, regardless 
of the direction of the FEL travel. 

The IR system was on the FEL for ap- 
proximately 6 months during the early 
spring and summer of 1984. Throughout 
the testing, the IR system revealed many 
weaknesses. Reports from the mine indi- 
cate that after 2 weeks of use, the sys- 
tem became sensitive to the vehicle's en- 
gine speed. Above 1,200 rpm it worked 
properly, but below this speed it had no 
sensitivity and false-alarmed. Dust ac- 
cumulation on sensor windows caused the 
sensors to register a loss of sensitiv- 
ity, but this condition was easily cor- 
rected by cleaning the windows. Despite 
these flaws, the principal operator of 
the FEL liked the system because it was 
useful in stockpiling operations and 
maneuvering near the highwall. The oper- 
ator also felt the system had prevented a 
couple of collisions. 

At the conclusion of the testing peri- 
od, an examination of the system revealed 
that the sensor leads between the control 
electronics and the sensors were worn and 
abraded in several places, causing short 
circuiting of the leads. This accounted 
for the poor operation of the system and 
indicated the need for better routing and 
securing of the leads. 

The Bureau has mounted similar STI sys- 
tems on smaller FEL's: a Clark 275 (fig. 
7) and a Caterpillar 980 (fig. 8). The 
installations were much the same as pre- 
viously described except for the physical 
placements of the sensors and control 
electronics, which were located according 
to the situation and available cab space. 
Flexible, watertight conduit was used in 



the Caterpillar 980 installation to pro- 
tect the leads where they ran between 
the cab and the sensor location. These 
systems were affected by dust, sunlight, 
and reflected light from white sand, mak- 
ing their overall performance marginal. 

Sear ch-Eye 

Another IR based system, known as 
Search-Eye, manufactured by Global Fabri- 
cations Co., Ltd., in Weston, Ontario, 
Canada, was also tested. This system was 
designed primarily for use on street and 
alley refuse haulage trucks. It operates 
on the same principle as the previously 
described STI system. Reflected IR light 
from the detected object is sensed and 
activates a warning buzzer and light. It 
is different from the STI system in its 
packaging and control electronics. The 
Search-Eye system uses three sensor- 
detectors spaced across the width of the 
vehicle. An IR beam is spread horizon- 
tally, and vertically (though to a lesser 
capacity), by two long, narrow, plastic 
lenses. This results in a wide field of 
coverage for each sensor [about 1 m (39 
in)], but severely shortens the range of 
detection to 1.1 m (3.7 ft). This system 
was checked in the laboratory and briefly 
in the field on a FEL, but its detection 
range was too short for use in large mine 
vehicles. 

ULTRASONIC SENSING SYSTEM 

Global Fabrications Co. also manufac- 
tures an ultrasonic-based object sens- 
ing system, under the trade name Sonic 
Radar. The principle on which the system 
operates involves the emission of an ul- 
trasonic sound burst, followed by detec- 
tion of a reflected energy wave returned 
to the source by contact with an object. 
The system is composed of four major 
parts: two front sensors, two rear sen- 
sors, control box, and alarm; however, 
the front sensors were not used in field 
testing because they do not affect the 
rear blind area for mining applications 
(fig. 9). 



11 



Electronic 
control box 



+ 24 V 




Rear-mount active sonic 
detectors (4) 



FIGURE 9. - Schematic diagram of an ultrasonic detection system. 



The Sonic Radar system is installed 
in much the same way as the IR systems, 
although the sensors require more room. 
Also, the sensors must not be mount- 
ed where the engine radiator cooling 
airstream can flow around them, as 
this airflow can cause the system to 
false-alarm. Figures 10 and 11 show one 



typical location for sensors on a FEL. 
The system operates from a 12- or 24-V dc 
electrical system. The sensors are in a 
14-gauge steel housing, with end brackets 
provided for swiveling the housing to aim 
the beam. Two systems were field tested, 
on a Caterpillar 988B loader and a Clark 
275C loader. (See table 1.) 



12 




FIGURE 10. - Ultrasonic detection system installed on a Clark 275C. 




FIGURE 1 1. - Close-up of ultrasonic sensors. 



13 



The system was connected to energize 
the rear sensors when the vehicle's en- 
gine was switched on. The sensors will 
detect any object up to 4.4 m (16 ft) 
away from the sensor. When an object is 
detected, a pulsating alarm sounds in the 
cab. When the object is within an ad- 
justable range of 0.9 to 2.7 m (3 to 9 
ft), the alarm will change to a constant 
tone. 

Though not specified as an option, 
an external solid-state alarm was con- 
nected to the control box, so that it 
would sound upon detection of an object. 
Both the external alarm and the in-cab 
alarm will sound when an object is sensed 
within the system's detection zone. 
Figure 12 depicts how the zone was mea- 
sured, with the black cord on the ground 
outlining the zone. The sonic wave 
coverage is actually somewhat conical 
near the FEL because of the separation 
between the right and left sensors. The 
coverage overlaps at 9 ft from the FEL 
when the transducer pairs are placed 4- 
1/2 ft apart; this produces a zone of 
nondetection near the FEL in the area 
that the spreading cones do not reach. 



The system detects objects about 1 ft 
above ground level when the full cone is 
developed 9 ft from the FEL. Beyond 9 
ft, the outline of coverage was well de- 
fined and rectangular in shape; overall 
the coverage area measured approximately 
11 by 17 ft. 

One of the two Sonic Radar systems cur- 
rently being tested is on a Clark 275, 
and the other is on a Caterpillar 988B. 
At last report, both systems were working 
well and physically withstanding the 
dusty and harsh mine environment. The 
mine managers like the systems because of 
the reduction of noise from the back-up 
alarm, as well as the positive safety 
warning given upon detection of an object 
in the rear blind area. 

DOPPLER RADAR SYSTEM 

A short-range Doppler radar system, 
manufactured by Con-Serv, Inc., Omaha, 
NE, was recently made available for test- 
ing. Called an "Electronic Mirror," it 
is a radar device that uses the Doppler 
shift principle to detect the presence of 
a moving target within its range. The 







FIGURE 12. - Measuring the detection zone. 



14 



system is made up of a transceiver, an 
antenna, an intermediate frequency ampli- 
fier, and an audiovisual alarm. 6 The 
transceiver consists of a Gunn diode 

"Con-Serv, Inc., has also manufactured 
similar Doppler radar devices for use on 
school buses. Such a device is credited 
with saving the life of a child in Val- 
dez, AK, in February 1986, when the buzz- 
er alerted the bus driver to the presence 
of the child "retrieving a football that 
had rolled under the bus." The school 
district had contacted Russell Griffin of 
the Bureau regarding acquisition of this 
safety equipment. This lifesaving inci- 
dent occurred just 1 day after the sensor 
was installed on the bus. 



mounted in a waveguide cavity, providing 
a transmitter, local oscillator, and a 
barrier mixer for the receiver. Output 
frequency is factory preset at 10.525 GHz 
and the power output is 5 mW. The rest 

of the circuitry is card-mounted and 
treated with conformal coating to prevent 
moisture and salt corrosion. 

These units are mounted in an environ- 
mentally sealed, high-impact, plastic 
housing (fig. 13). A splashproof , four- 
conductor connecter is mounted on the 
rear of the unit to provide for power and 
connections to the audiovisual alarm and 
the vehicle's backup alarm. Two differ- 
ent types of antennas are available for 
use on various sized vehicles. They are 
constructed of diecast and machined alu- 








FIGURE 13. - Close-up of a Doppler radar detection system. 



15 




Doppler radar unit 
recessed in grill 



Extended 

vehicle 

detection 

zone 



12-0" ■ 



9'-0" 



1 ll'-O" 



2l'-0" 



FIGURE 14. - Detection zone of a Doppler radar 
system. 



aluminum and flange mounted directly to 
the transceiver waveguide cavity, elimi- 
nating the effects of noise and false re- 
sponses. Antenna range is adjustable to 
accommodate the needs of different-sized 
vehicles for blind area coverage. An in- 
tegrally molded projection on the hous- 
ing provides a means of mounting the unit 
on a universal bracket, which is then 
mounted on the rear of the vehicle. 

A Clark 275C FEL, a Caterpillar 992 
FEL, and a Caterpillar 988 FEL were used 
during several months of testing the 
unit for durability and functionality. 
Figure 14 diagrams the area coverage in 
which an object can be sensed. This area 
will vary depending on the height , angle 
of declination, and sensitivity adjust- 
ment of the radar unit. 

A similar unit was also tested at the 
Bureau's Twin Cities facility by a con- 
tractor using a Caterpillar 910 FEL. Af- 
ter installation, the operator backed up 
towards an anthropomorphic dummy posed in 
a seated position, placed on the ground 
(fig. 15). The operator received a warn- 
ing at the 12- to 18-ft range (depending 
on the angle of approach) , and was able 
to stop in time on each trial. The range 
of the unit is adjustable and will easi- 
ly accommodate any FEL. One specially 
designed system was capable of detecting 
a person at 28 ft but was not field 
tested. 

The units have stood up well in the 
mine environment and operated satisfac- 
torily. In one case, the mine ordered a 
unit on its own to equip a second FEL. 
The feeling seems to be universal that 
such devices will not only protect the 
vehicle from rear collisions, but also, 
when connected to the backup alarm re- 
quired on the vehicle, enhance safety 
aspects by reducing the amount of un- 
necessary noise. 

POSSIBLE HARDWARE IMPROVEMENTS 

Each of the tested systems has limita- 
tions of width or distance coverage. In 
the case of the IR systems, there is a 
tradeoff between projection distance and 
width of beam with a given intensity. 
This is also true with the ultrasonic 



16 




FIGURE 15. - Testing a Doppler radar system with an anthropomorphic dummy. 




FIGURE 16. - Prototype synthesized voice warning syster 



17 



system, but sonic waves naturally dis- 
perse more and are difficult to project. 
As efficiencies in light-emitting diodes 
and sonic transducers are improved, along 
with the development of different driv- 
ing and receiving circuit techniques, 
both range and width of coverage should 
increase. 

One easily implemented improvement in 
any alarm system would be the substitu- 
tion of voice warning output for audio 
alarm output. Voice synthesis and dig- 
itizing has made great strides and 



continues to come down in cost. There is 
also evidence that voice warning is 
more effective in attracting attention. 
Such a prototype system was fabricated 
in the laboratory to demonstrate the 
applicability of the improvement. (See 
figure 16.) 

Another improvement would be to include 
a test switch that could power systems 
for preshift vehicle inspection and daily 
testing of the alarm. This would allow 
checking the system without starting the 
FEL and placing it in reverse gear. 



INSTALLATION TIPS 



During the course of the installation 
and field trials, several techniques and 
procedures were found to be of help in 
maintaining the operating systems. The 
following tips are listed to aid in the 
field installation of these systems. 

1. Protect long runs of wire and cable 
from abrasion by installing them in 
conduit. If possible, it is best to fol- 
low existing runs of hoses, wires, or ca- 
bles. The liquidtite type of conduit is 
easy to use. Once the conduit is in 
place, it can be fastened with wire 
ties and the necessary wires and cables 
pulled through it with the aid of a 
"snake." Each end of the conduit should 
be finished off by using a bushing. Al- 
so, run wire, cable, or conduit, to allow 
for maintenance and removal of engine 
components with as little disturbance as 
possible. 

2. Use some type of environmentally 
protected housing for the electronics. A 
NEMA 4- or 12-type box is probably the 



easiest to obtain. If possible, locate 
the control box inside the cab. 

3. Physically isolate components from 
vibration effects if the manufacturer has 
not done so. 

4. Locate and mount the sensors in 
such a way as to allow for easy re- 
moval for engine maintenance access. 
Also, make sure they cannot easily be 
knocked off or broken during machine 
operation. 

5. Keep ultrasonic type sensors away 
from the engine cooling airstream. Mount 
the IR types where the least amount of 
dust can accumulate on them. In both 
cases, allow the sensor a clear view to 
the rear area. 

6. Use an appropriately sized fuse in 
the power connection to the system. 

7. Consult an individual familiar with 
the electrical wiring layout of the ve- 
hicle to help determine the best place to 
connect for primary power. 



DISCUSSION 



INFRARED SYSTEMS 

IR object detection systems do not per- 
form well in mines. False alarms can be 
triggered by bright sunlight and reflec- 
tions of the mine ground (such as white 
sand) . Their range depends upon the 
reflectivity of the detected object, 
which is variable due to factors like 
soft clothing, hard hats, reflective 
tape, steel machinery, and reflectors on 
machinery. Because of the narrow beam 



pattern, arrays of sensors must be 
employed and reflectivity within the mine 
must be made more uniform. Increasing 
the number of sensors increases the cost 
and system complexity yet decreases 
the system's reliability. The narrow 
beam from a sensor, mounted above the 
rear bumper for protection, does not have 
the vertical coverage to detect a per- 
son sitting or kneeling on the ground. 
Detection of this type of object would 
require more sensors angled downwards 



18 



toward the ground. The addition of re- 
flectors to clothing and equipment is 
necessary to insure uniform reflectivity 
in a mine. This would promote a uniform 
detection range, but it would be expen- 
sive to initiate and maintain. For these 
reasons, 1R technology is the least at- 
tractive method of detecting objects in 
the rear blind area. 

ULTRASONIC SENSING SYSTEMS 

Ultrasonic object detection systems 
utilize transverse mechanical waves at 
the low end of the ultrasonic spectrum. 
The Polaroid transducer operates with 50, 
53, 57, and 60 kHz pulses and has a beam 
angle of 15° from the electrostatic com- 
bination transmitter-receiver. The Sonic 
Radar unit uses a separate piezoelectric 
transmitter and receiver operating at 
32.8 kHz, with a beam angle measuring ap- 
proximately 34°. This system provided a 
nearly rectangular detection zone, but 
its range of only 17 ft is too short for 
large machines. 

Another problem with ultrasonic systems 
is that the wave velocity is considerably 
slower than IR light or microwaves (used 
with Doppler radar systems). Several 
waves must be sent, received, and com- 
pared in order to insure that the sample 
of wave travel time is accurate. At a 
velocity of 1,090 ft/s in air, a wave re- 
quires 1/1090 s times 2 ft, or 1.83 ms , 
for each foot of detection range (travel 
distance to and from target) . If many 
waves are sampled (Polaroid uses fifty- 
six 1-ras pulses) , the time between ini- 
tial detection and operator warning or 
system reaction time may become great 



enough to preclude warning the operator 
in time to stop the vehicle. 

In field tests, the Sonic Radar's sys- 
tem reaction time was approximately 0.5 
s for an object at the edge of the detec- 
tion zone. This long system reaction 
time, coupled with a short detection 
range, limits the use of this type of 
system to smaller FEL's operating at low- 
er speeds. 

DOPPLER RADAR SYSTEMS 

Doppler radar systems use the Doppler 
frequency shift principle to detect ob- 
jects. This requires relative motion be- 
tween the system and the object being de- 
tected. The beam pattern is controlled 
by the design of the antenna. The detec- 
tion range is controlled by the power 
output, sensitivity, and shape of the an- 
tenna, as well as by the "radar profile," 
or ability to reflect microwaves, of the 
object to be detected. Virtually any de- 
tection zone range and shape can be pro- 
duced; however, the radar profile of ob- 
jects in mines is variable. In general, 
larger, more reflective objects can be 
detected at longer ranges. In-mine tests 
demonstrated that a system capable of de- 
tecting a person at a distance of 20 ft 
would detect a small car at 40 ft and a 
large metal building at several hundred 
feet. In most operations, detection of 
large objects is not a problem, as FEL's 
usually back away from stockpiles and 
mine structures such as hoppers. 

The Doppler radar systems are not 
affected by lightning, rain, fog, snow, 
or wind, as were the other types of 
systems . 



CONCLUSION 



The Bureau of Mines experimented with 
IR light, ultrasonic wave, and Doppler 
radar technology to develop a system 
capable of detecting an object in the 
rear blind area of mobile mining equip- 
ment. This system sounds an alarm only 
when a collision hazard exists, reduces 



the semicontinuous noise of backup 
alarms, and eliminates the false alarm 
aspect of current backup alarms. Of the 
three technologies, Doppler radar proved 
to be the best compromise because of its 
immunity to various weather conditions. 



APPENDIX.— SOURCES OF EQUIPMENT 



19 



Scientific Technology, Inc. 

1201 San Antonio Road 

Mountain View, California 94043 



Con-Serv, Inc. 
3801 Dahlman Avenue 
Omaha, Nebraska 68107 



Global Fabrications, Ltd 

4 Twyford Road 

Toronto 

Ontario, Canada M9A1V7 



Polaroid Corporation 
Ultrasonic Components Group 
119 Windsor Street 
Cambridge, Massachusetts 02139 



U.S. GOVERNMENT PRINTING OFFICE: 1986-605 017/40,029 



INT.-BU.O F MINES,PGH.,P A. 28 27 1 



U.S. Department of the Interior 
Bureau of Mines-Prod, and Distr. 
Cochrans Mill Road 
P.O. Box 18070 
Pittsburgh. Pa. 15236 



OFFICIAL BUSINESS 
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